What prevented India/China from being Christianized or Islamicized?

[Kamala's side hoe]

It’s rather self-explanatory, but I’ll explain to make it clearer for you. The majority of India did not convert to Islam, it was just localized mostly to the parts that became Pakistan and Bangladesh. Not even close to the entire country. Why wasn’t India “Islamicized” like Iran/Persia, Egypt, Mesopotamia, Syria, Central Asia, North Africa, etc?

The politically correct answer is that like Medieval China, Medieval India was already too old and too populous of a civilization to have its indigenous religious traditions and social order completely uprooted by Islam (or Christianity, in the case of Medieval China).

I would instead submit that we’re well on our way to a tri-polar world where all those 20th and early 21st-century foes actually collapse into a single sphere characterized more by its deeply intertwined roots than tiny distinctions among its branches. Instead, the greatest contrasts I foresee the world grappling with in decades to come include the deep differences among the two cultural giants who effectively sat out the global 20th century. And not just each of their distinct contrasts with the West.

“The West and the Rest” has an irresistible ring, but when you get down to it, the incommensurability of Indian culture to that of China yawns just as unfathomably, or more so than that between the West and either Asian giant. For my money, the irreconcilable cultural poles the remaining century will bring into focus are these three ancient traditions: the broader Abrahamic West, the Chinese sphere and the Indian one.

We live in a globalized world. But our hyperconnected landscape has notably not produced a flat homogeneity of values. India, China and the West, which encompasses Christian Europe and the Muslim Near East, continue to serve as distinct and unique cores of human civilization. These three great pillars of human tradition reflect profoundly different values and orientations in their elite cultures. We have pragmatic China, philosophical India and a broader West threading a path between the two. Most remaining civilizations can easily be understood as both derivative and a synthesis of these traditions. Latin America is broadly Western, with indigenous American and African elements. Southeast Asian nations balance Indian, Chinese, Islamic and indigenous influences.

[Kamala's side hoe]

The local Hindu leaders managed to have autonomy and due to their numbers and the diversity of religious thought survive these centuries while resisting the central government. Same in China.

Yeah, no ruling power over any part of "India" has ever successfully supplanted Hindu aristocracy.  There is a massive difference between the Spanish completely wiping out indigenous elites in the Americas and the relative freedom of expression awarded to India over the centuries, as far as the effect it has on the long term cultural shifts.

And there are very important reasons for this "relative freedom of expression".

The really high preexisting population density vs. the rest of the world seems like the obvious answer?

[Kamala's side hoe]

It’s rather self-explanatory, but I’ll explain to make it clearer for you. The majority of India did not convert to Islam, it was just localized mostly to the parts that became Pakistan and Bangladesh. Not even close to the entire country. Why wasn’t India “Islamicized” like Iran/Persia, Egypt, Mesopotamia, Syria, Central Asia, North Africa, etc?

The politically correct answer is that like Medieval China, Medieval India was already too old and too populous of a civilization to have its indigenous religious traditions and social order completely uprooted by Islam (or Christianity, in the case of Medieval China).

What is the politically incorrect answer? Also, couldnt i say that zoroastrian persia was as grand and old of a civ as china/india and yet it was Islamized

It would probably be a Leipforsaken cocktail of Hindutva talking points, CCP propaganda, and BIPOCel keyboard warriors rambling about "yt fragility".

https://en.wikipedia.org/wiki/Estimates_of_historical_world_population#By_world_region

The real answer has do to with how much of the world's population was concentrated in India and China 1000 to 2000 years ago. China alone had something like 25%, the Indian subcontinent had 25-30%. All of Europe and West Asia combined had only 20-23%.

This is basically why Muslim conquerors from West and Central Asia were never able to impose "wholesale, top-down Muslim" rule throughout South Asia as laddicus finch put it, and had to delegate many powers to local rulers.

[Kamala's side hoe]

What prevented India/China from being Christianized or Islamicized, unlike most of the world?

 Isn't the premise somewhat flawed in that India is over 1/8 Muslim, With multiple States having a significant Muslim majority?  

You're not wrong- there are 2 >50% Muslim states (Jammu & Kashmir, Lakshadweep)- although they make up like 1% of India's population combined. Assam (34%) and Kerala (27%) also have large Muslim minorities.
2021 source- chart data is from 2011 census



And yeah, as TDAS04 said the Christian majority/plurality states are all on the border with Myanmar.